Preheating screws have been used for the preheating and intensive mixing of petroleum coke for the manufacture of anode carbons. Preheating screws having rotating hollow screws through which a liquid heat-exchange medium flows, are described in the brochure "Hollow-Screw Heat Exchangers," 1571 d/3.88, of the firm of Lurgi, Frankfurt am Main. Typically, the heat exchanger liquid flows through the hollow shaft first and then, in the vicinity of the product discharge from the preheating screw, enters the last hollow helical flight and then flows through the helical flight in countercurrent to the product being transported in the screw trough.
From the processing standpoint, this known preheating screw has the disadvantage that the heat-transfer medium cools as a result of giving up heat to the product being heated, by which means an axial temperature gradient, appropriately oriented depending on the flow direction of the heat-transfer medium, comes into being. With regard to fabrication cost, it is disadvantageous that the heat-transfer medium is introduced into the rotating screw shaft from outside and must also be removed from the rotating screw shaft to the outside, for which purpose mechanically complicated and maintenance-intensive rotary couplings, called "seal caps" in the company brochure cited above, are required.